Group-transfer reactions of a cationic iridium alkoxycarbene generated by ether dehydrogenation

Scott M. Chapp and Nathan D. Schley*

Department of Chemistry, Vanderbilt University, Nashville, Tennessee 27235 United States

ABSTRACT: Despite broad interest in metal carbene complexes, there remain few examples of catalytic transformations of ethers

that proceed via alkoxycarbene intermediates generated by α,α-dehydrogenation. We demonstrate that both neutral and cationic

alkoxycarbene derivatives are accessible via ether dehydrogenation at a PNP(iPr)4 pincer-supported iridium complex (PNP(iPr)4 =

2,6-bis((diisopropylphosphino)methyl)pyridine). Both cationic and neutral alkoxycarbene complexes undergo group transfer imina-

tion with azides, with the cationic derivative serving as a more efficient catalyst for cyclopentyl ether imination. Mechanistic stud-

ies support an iridium(I)dinitrogen complex as the resting state in the dark and a role for light-promoted N2 dissociation. Isoamyl

nitrite and phenyl ethyl ketene are also found to engage with the cationic alkoxycarbene complex in formal alkoxide- and O-atom

transfer reactions respectively. In the former case an isolable dialkoxyalkyliridium complex is obtained, representing only the sec-

ond example of a structurally-characterized dialkoxyalkyl complex of a transition metal.

Introduction

The selective activation and functionalization of inert C-H

bonds by homogeneous metal complexes has been an area of

significant interest since pioneering studies on alkane C-H

oxidative addition and alkane dehydrogenation.1-4 These foun-

dational stoichiometric systems led the way to early examples

of catalytic alkane transfer dehydrogenation.5-9 Further devel-

opment of alkane dehydrogenation over the intervening dec-

ades has revealed design criteria that favor neutral, electron-

rich and thermally-stable pincer iridium complexes analogous

to those initially applied to alkane dehydrogenation by Kaska

and Jensen in 1996.10-14

In contrast to the large body of work on alkane dehydro-

genation, the dehydrogenation of compounds bearing heteroa-

tom functionality has seen considerably less attention. Jensen

and Kaska reported the transfer dehydrogenation of THF to a

mixture of furan and dihydrofurans15 and Goldman has report-

ed the ,-dehydrogenation of alkyl amines.16-17 More recent-

ly, Brookhart18 and Huang19 demonstrated that neutral PCP

pincer-supported iridium complexes catalyze the transfer de-

hydrogenation of alkyl ethers to vinyl ether products. The ,-

dehydrogenation of amines and ethers is complicated by com-

peting ,-dehydrogenation to give aminocarbene and

alkoxycarbene complexes respectively. The formation of iridi-

um alkoxycarbene complexes via ether dehydrogenation and

their reactivity has been studied extensively both by Carmona

and by Whited and Grubbs.20-21 Grubbs’ work demonstrated

that neutral iridium alkoxycarbenes undergo group-transfer

reactions with heterocumulenes and certain 1,3-dipolar rea-

gents including CO2, phenyl isocyanate and adamantyl azide

(Ad-N3). These transformations were proposed to occur via

initial [2+2] cyclization to give a 4-membered iridacyclic in-

termediate. This reactivity is distinct from that of canonical

group 6 alkoxycarbenes, and argues for a significant role for

the high-lying, filled dz2 orbital in reactions of d8 alkoxycar-

benes.22

Figure 1. Recent ether-derived alkoxycarbenes.

In this context our group has been exploring the chemistry

of cationic bis(phosphine)iridium alkoxycarbene complexes

with the aim of developing systems with reduced metal-

alkoxycarbene backbonding and enhanced electrophilicity

relative to neutral variants. Prior to our studies, a single exam-

ple of THF dehydrogenation to give a cationic iridium

alkoxycarbene over a period of days had been reported by

Schneider (Figure 1).23 We have since demonstrated facile

formation of cationic alkoxycarbene complexes via ether de-

hydrogenation in Lewis base-directed intra- and intermolecu-

lar examples, with evidence for reversible -hydride insertion

and C-O bond cleavage reactions in certain cases.24-25 Despite

the body of work on the generation of alkoxycarbene com-

plexes via ether dehydrogenation, few stoichiometric trans-

formations and only a single catalytic reaction have been re-

ported.21, 26-27 We now report the synthesis of a cationic pincer-

supported Ir(I) alkoxycarbene complex generated by ,-

dehydrogenation of cyclopentyl methyl ether (CPME) and

demonstrate its reactivity in atom- and group-transfer reac-

tions with an alkyl azide, a ketene and an alkyl nitrite. This

complex serves as a catalyst in ether imination, showing that

2

cationic iridium complexes are sufficiently competent at ether

dehydrogenation to support catalytic transformations.

Results and Discussion

Treatment of commercially-available [(cod)2Ir]BArF4 with

2,6-bis((diisopropylphosphino)methyl)pyridine (L1) gives

complex 1 (eqn. 1) which undergoes hydrogenation to an irid-

ium dihydride complex 2 (eqn. 2). Dehydrogenation of 2 with

tert-butylethylene in CPME yields a product with a single 31P{1H} NMR signal and a broad, downfield-shifted 1H reso-

nance at 13.5 ppm. This signal is in the range expected for an

alkoxymethylidene Ir=CHOR resonance,28-30 which along with

the appearance of a 13C{1H} resonance at 234.7 ppm led us to

assign the product as cationic alkoxycarbene complex 3, a site

of CPME activation distinct from the 3,4-dehydrogenation

observed in a related non-pincer complex.31 Numerous at-

tempts at confirmation of this assignment through X-ray crys-

tallography ultimately led to a structure solution for 3 from

samples crystalizing as 3-component merohedral twins in the

P21 space group (Figure 2). The unambiguous characterization

of 3 represents one of only a few cationic iridium alkoxycar-

bene complexes generated by ether dehydrogenation.23-25, 32

Figure 2. ORTEP diagrams of 1 (left) and 3 (right) shown at

50% probability. Full disorder models are omitted for clarity.

Selected bond distances in 3 (Å): Ir=C 1.880(12), Ir-Npy

2.179(8), O-Ccarbene 1.342(16).

Work by Milstein has shown that metal-bound

bis(phosphinomethyl)pyridines like L1 can undergo deproto-

nation at the -phosphino methylene position to give a formal-

ly monoanionic PNP ligand.33 Treatment of 3 with potassium

tert-butoxide gives the neutral alkoxycarbene complex 4 (eqn.

3), which can be distinguished by a shift of the Ir=CHOR 13C

resonance from 234.7 to 221.5 ppm. The bond metrical pa-

rameters obtained from the crystal structure of 4 show signifi-

cant Kekulé distortion of the pyridine moiety and contraction

of one exocyclic C-C bond consistent with those observed in

other systems (Figure 3).34-35 The Ir=Ccarbene bond length in 4 is

1.882(3) Å, which is indistinguishable from the bond length of

1.880(12) Å in 3 and similar to that of a neutral (PNP)Ir(I)

alkoxycarbene complex reported by Grubbs (1.884(4) Å)30 as

well as a cationic iridium(I)alkoxycarbene (1.912(4) Å) we

have previously reported.24 Thus the iridium carbene bond

length in 3 and 4 appear insensitive to the net ionic charge. In

contrast, these values are shorter than those observed for a

cationic iridium(III) alkoxycarbene reported by Schneider

(1.938(3) Å)23 as well as a trio of cationic iridium(III)

alkoxybenzylidenes we have previously reported (1.997(3),

1.997(8) and 2.001(2)).24-25 These observations suggest that

iridium alkoxycarbene bond lengths are most sensitive to the

formal oxidation state.

Figure 3. ORTEP diagram of 4 shown at 50% probability.

Selected bond distances (Å): Ir=Ccarbene 1.882(3), Ir-Npy

2.096(3). O-Ccarbene 1.342(4).

The structural similarity of complex 4 to the family of neu-

tral PNP(Ir) alkoxycarbenes prepared by Whited and Grubbs

via ether dehydrogenation encouraged a closer comparison of

the two systems. In particular, we were interested in whether

the reactivity of the neutral complex 4 towards alkyl azides

would match that observed by Grubbs for the analogous com-

plex shown in Figure 1. Additionally, cationic derivative 3

could be compared directly to 4 in order to ascertain whether

the net ionic charge modulates the reactivity of alkoxycar-

benes towards group-transfer reactions.

Indeed, both complexes 3 and 4 react rapidly with Ad-N3 at

room temperature to give N-adamantyl formimidate 5 and the

corresponding Ir-N2 complexes 6 and 7 respectively (eqns. 4

and 5). N2 binding can be confirmed by analysis of their infra-

red spectra, which show an Ir-N2 band at 2141 cm-1 for 6 and

2076 cm-1 for complex 7. For comparison, a neutral (PNP)IrN2

complex reported by Grubbs absorbs at 2067 cm-1, demon-

strating a clear decrease in Ir-N2 backbonding for the cationic

variant 6. The assignment of 6 and 7 was confirmed by single

crystal X-ray diffraction of both species (Figure 4) and by

conversion of 6 to 7 on treatment with potassium tert-

butoxide.

3

Figure 4. ORTEP diagrams of 6 (left) and 7 (right) shown at

50% probability. Selected bond distances (Å), Complex 6: Ir-

N2 1.915(5), Ir-Npy 1.994(5), N≡N 1.105(8). Complex 7: Ir-N2

1.881(5), Ir-Npy 2.053(4), N≡N 1.083(7).

An analogous azide-alkoxycarbene group-transfer reaction

has been used previously as part of a synthetic cycle for the

imination of ethers to formimidates. The incompatibility of the

system with excess azide reagent initially precluded the devel-

opment of a true catalytic reaction, but through stepwise addi-

tion under photolysis, 4 turnovers could be obtained over sev-

eral days.26 These conditions were subsequently improved

through the slow addition of Ad-N3 over 30 hours, leading to a

catalytic system capable of 10 turnovers.21, 27

Table 1. Evaluation of Reaction Conditions

Entry Catalyst

(mol %)

Temp.

(C)

Time

(hr.)

Light

source Yielda

1 3 (10%) 23 22 blue 0.4%

2 3 (10%) 90 22 none 0.8%

3 3 (10%) 90 22 blue 83%b

4 3 (5%) 90 22 blue 39%b

5 3 (10%) 80 22 blue 70%b

6 4 (10%) 90 22 blue 29%b

7 3 (10%) 90 1 blue 68%b

8 1 (10%) 90 22 blue 84%b

a NMR yield. b Average of two experiments.

By comparison, cationic complex 3 serves as a competent

catalyst for the group-transfer imination of CPME under batch

conditions without requirement for slow azide addition. Under

optimized conditions under blue light irradiation we observe

8.3 turnovers to give 83% yield of formimidate 5 (Table 1,

entry 3). The majority of turnovers occur within the first hour

(entry 7), demonstrating rapid catalysis without the sensitivity

to excess azide observed in the neutral Grubbs system. In a

series of experiments we found that 3 reacts productively with

Ad-N3 to give 5 at 23 C, but that a reaction temperature of

90 C under blue light irradiation is required for productive

catalysis. When a catalytic reaction is examined in the dark by 31P{1H} NMR after exposure to blue light at 90 C for 5 or 15

minutes, complex 6 appears to be the major metal-containing

species in solution. This observation supports a role for light

in N2 dissociation and is consistent with 6 being on-path in-

termediate in catalysis (Scheme 1). Although 3 closely match-

es the total turnover number achieved by the Grubbs system,

the tolerance of batch conditions rather than a requirement for

azide slow addition is a marked improvement. In contrast, the

neutral complex 4 achieves only 2.9 turnovers under compara-

ble batch conditions (entry 7). The neutral N2 complex 7

shows sensitivity to excess Ad-N3, degrading to several uni-

dentified species over minutes to hours under irradiation. In

the case of the catalysis by the cationic complex 3, total turno-

ver numbers appear to be limited by catalyst degradation, as 31P{1H} NMR analyses show numerous unidentified products

after cessation of catalysis.

The success of complex 3 as an ether imination catalyst in-

spired us to examine the precursor, complex 1 for the same

transformation. Dissociation of 1,5-cyclooctadiene would pro-

vide access to a three coordinate 14 e− Ir(I) fragment A

(Scheme 1) analogous to those proposed as reactive interme-

diates in both -C-H activation of CPME and C-H activation

of alkanes.12-14, 23, 36 Under our optimized conditions complex 1

shows comparable activity to 3 with 8.4 TON (84%) (Table 1,

entry 8). This observation should simplify future reaction de-

velopment since derivatives of ligands related to L1 can be

accessed in one step from commercially available iridium

starting materials. Indeed the tBu analogue of 1 has been pre-

viously reported.37

A proposed mechanism for catalytic CPME imination by 3

is given in Scheme 1. Reaction of 3 with Ad-N3 could proceed

via either initial [2+2]26 (B) or [2+3]38 cycloaddition, after

which extrusion of formimidate 5 would give N2 complex 6.

Light-promoted dissociation of dinitrogen would give the 14

e− Ir(I) fragment A, which is likely the species responsible for

CPME activation to regenerate 3.

Scheme 1. Proposed catalytic cycle for group transfer imina-

tion of CPME. Both [2+2]26 (B) and [2+3]38 azide-carbene

cycloaddition mechanisms have been previously proposed.

In total, the stoichiometric reactivity of alkoxycarbene com-

plexes 3 and 4 with Ad-N3 closely mirrors observations made

by Whited and Grubbs. In our case, complex 3 was found to

serve as a catalyst for group transfer imination of CPME with-

4

out requirement for portionwise or slow addition of azide,

while the neutral complex 4 appears to share the Grubbs sys-

tem’s reported sensitivity to excess azide.26

As complexes of square planar d8 metal ions, 3 and 4 bear a

filled high-lying dz2 orbital which has been implicated in so-

called Roper-type carbene chemistry of which group-transfer

reactions of azides represent one example.21-22, 39-40 Other elec-

trophiles including CO2, carbonyl sulfide, and phenyl isocya-

nate have been demonstrated to give formate esters, thiofor-

mates and formimidates, respectively.22, 30 We suspected that

other substrates might undergo similar group-transfer reac-

tions via what has been proposed as an initial [2+2] cycloaddi-

tion22 to alkoxycarbene 3, and identified diazoalkanes, ke-

tenes and alkyl nitrites as possible candidates for C-C or C-O

bond-forming chemistry. Just as alkyl azides are observed to

transfer a formal nitrene equivalent, we hypothesized that di-

azoalkanes and alkyl aryl ketenes41-45 might serve as carbene

equivalents to give the products of formal carbene-carbene

cross-coupling.

Surprisingly, complex 3 is largely unreactive towards either

one equivalent or an excess of trimethylsilyldiazomethane at

room temperature. While elevated temperatures or irradiation

with blue light did lead to partial consumption of 3, the N2

adduct 6 is observed only in trace quantities and no organic

product of carbene transfer was detected. In contrast, treat-

ment of 3 with 25 equivalents of phenyl ethyl ketene gives

cyclopentyl formate and two new iridium-containing species

in a ratio of 95:5 by 31P{1H}NMR (eqn. 6).

Figure 5. ORTEP diagram of 8 shown at 50% probability.

The full disorder model and anion are omitted for clarity. Se-

lected bond distances (Å): Ir=Cα 1.828(5), Cα=Cβ 1.304(6),

Ir-Npy 2.124(4).

We have characterized the minor product as the Ir(I)-CO

complex 9 by doping with an authentic sample of 9 generated

independently46 (see the supporting information). The major

product was separable by crystallization and found to be the

unexpected cationic iridium vinylidene complex 8 (Figure 5).

Thus, phenyl ethyl ketene appears to serve as an oxygen atom

donor rather than a carbene equivalent. This outcome is inter-

esting when considered alongside the reported reactivity of

phenyl isocyanate with a related neutral alkoxycarbene,30

which serves as a nitrene source despite the electronic simi-

larity of ketenes and isocyanates.47 Ketenes undergo thermal

[2+2] reactions with a variety of substrates including simple

olefins, but such reactivity engages the C=C fragment of the

ketene moiety.48 In our case the observed group transfer reac-

tion of phenyl ethyl ketene likely requires that initial [2+2]

cycloaddition occur via the C=O fragment, implicating the

pair of stepwise or concerted, asynchronous nucleophilic addi-

tions shown in Scheme 2 rather than a concerted, synchronous

[2+2] process. A closely-related mechanism has been pro-

posed for a series of oxygen atom transfer reactions of an iso-

lable niobocene ketene complex,49-50 demonstrating the O-

nucleophilicity of α-metalloketenes. The structure of a (diphe-

nylketene-κ2O,C1)iridium(I) complex reported by Grotjahn is

also consistent with this proposal, though O-atom transfer

from a diphenylketene-κ2C1,C2 adduct may also be possible.51

Scheme 2. Proposal for ketene O-atom transfer

Previous studies by Whited and Grubbs were limited to

group- or atom-transfer reactions of heterocumulenes, but the

possibility that 3 might be capable of non-concerted group

transfer reactions encouraged us to explore other electrophiles.

One promising reagent – isobutyl nitrite was found to react

with 3 to give a single new complex with a major 31P{1H}

NMR signal at 34 ppm in 92% yield. We have characterized

this species as the iridium dialkoxyalkyl nitrosyl complex 10

resulting from formal alkoxide group-transfer and scission of

the N-O bond. Transition metal dialkoxyalkyls are proposed as

tetrahedral intermediates in the alkoxide exchange of

alkoxycarbene complexes, however there is only a single re-

port of a transition metal dialkoxyalkyl complex in the Cam-

bridge crystallographic database. In that case, nucleophilic

attack of NaMn(CO)5 on di(phenoxy)chloromethane gave the

corresponding manganese diphenoxyalkoxyalkyl,52 making 10

the only isolable dialkoxyalkyl generated by alkoxide transfer

to an alkoxycarbene.

The solid-state structure of 10 shows a square pyramidal

complex containing a bent nitrosyl with an Ir-N-O angle of

123.0(5) and a vacant site trans to the nitrosyl ligand, an ar-

rangement shared by all other reported bent, 5-coordinate irid-

ium nitrosyls.53-58 Though we have been successful in charac-

terizing 10 in the solid state by both single-crystal X-ray dif-

fraction (Figure 6) and elemental analysis, the instability of 10

in solution has precluded the collection of high quality 1H and 13C{1H} NMR data. Nonetheless, the dialkoxyalkyl 1H reso-

nance can be identified as a triplet occurring at 7.18 ppm with 3JHP = 4.7 Hz, which collapses to a broad singlet on 31P decou-

pling. This is in good agreement with the reported manganese

dialkoxyalkyl which resonates at 7.38 ppm. There is no re-

ported 13C chemical shift for the manganese dialkoxyalkyl,

5

however an HSQC experiment with 10 shows a correlation

with a 13C resonance at 94.2 ppm, which aligns well with an

iridium hydroxyaminoalkyl reported to resonate at 96.9 ppm.59

The conversion of 3 to 10 presumably occurs via initial

binding of Ir to the electrophilic N atom of isobutyl nitrite.

This binding mode has been inferred previously on the basis

of IR and NMR data at complexes of Ru60 and Ir,61 and shown

in a single crystallographically characterized example on Pd.62

Subsequent alkoxide transfer to the alkoxycarbene would give

10. A related reverse reaction – the formation of an N-

coordinated alkyl nitrite via alkoxide attack at a metal nitrosyl

has been previously observed at Ir.61

Figure 6. ORTEP diagram of 10 shown at 50% probability.

The full disorder model and anion are omitted for clarity. Se-

lected bond distances (Å) and angles (°): Ir-Cα 2.040(6), Ir-Npy

2.167(5), O-Cα-O 110.2(5), Ir-N=O 123.0(5).

Conclusion

In summary, we report the synthesis of cationic and neutral

PNP(iPr)4 iridium alkoxycarbene complexes via transfer dehy-

drogenation of CPME. Both alkoxycarbene complexes react

with Ad-N3 to form the corresponding Ir-N2 complex and a

formimidate resulting from formal nitrene transfer to the

alkoxycarbene. This reactivity mirrors observations by Whited

and Grubbs on a related neutral iridium alkoxycarbene system.

We have translated this stoichiometric reactivity to catalysis in

both the cationic and neutral alkoxycarbene cases, however

the cationic complex 3 shows superior performance. Under

optimized batch conditions 3 displays 7 TON within the first

hour, comparing favorably against the previously reported

system which requires slow addition of azide over more than

24 hours for comparable turnover numbers. The precursor

complex [(PNP(iPr)4)Ir(2-cod)]BArF4 (1) is also found to

serve as an active catalyst with similar performance.

In addition, we have further expanded the scope of reagents

which undergo atom or group-transfer reactions with iridium

alkoxycarbenes to include phenyl ethyl ketene and isobutyl

nitrite. These experiments have led to the isolation of a ketene-

derived iridium vinylidene complex and the first late transition

metal dialkoxyalkyl complex to be structurally characterized.

The reactivity of 3 with phenyl ethyl ketene and isobutyl ni-

trite is suggestive of stepwise processes rather than a concert-

ed [2+2] process. These results add to our understanding of

alkoxycarbene complexes of d8 metal ions and show promise

for future development for new functionalization reactions of

ethers that proceed through alkoxycarbene intermediates.

ASSOCIATED CONTENT

The Supporting Information is available free of charge at the ACS

publications website at “http://pubs.acs.org.”

Experimental procedures, X-ray crystallographic data,

and compound characterization data,.

AUTHOR INFORMATION

Corresponding Author

*E-mail for N.D.S.: nathan.schley@vanderbilt.edu.

ORCID

Scott M. Chapp: 0000-0001-6293-7948

Nathan D. Schley: 0000-0002-1539-6031

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

Funding from Vanderbilt University is gratefully acknowledged.

This material is based upon work supported by the National Sci-

ence Foundation under grant no. CHE-1847813. Acknowledg-

ment is made to the Donors of the American Chemical Society

Petroleum Research Fund for partial support of this research.

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8

For Table of Contents Only

Synopsis

We report the synthesis of a cationic pincer-supported Ir(I)

alkoxycarbene complex generated by ,-dehydrogenation

of cyclopentyl methyl ether and demonstrate its reactivity

in atom- and group-transfer reactions with an alkyl azide, a

ketene and an alkyl nitrite. This complex serves as a cata-

lyst for ether imination, showing that cationic iridium com-

plexes are competent to support catalytic transformations

of ethers that proceed through alkoxycarbene complexes.